[PATCH] xfs: fix data races in inode->i_*time

[Qian Cai]

inode->i_*time could be accessed concurrently. The plain reads in
xfs_vn_getattr() is lockless which result in data races. To avoid bad
compiler optimizations like load tearing, adding pairs of
READ|WRITE_ONCE(). While at it, also take care of xfs_setattr_time()
which presumably could run concurrently with xfs_vn_getattr() as well.
The data races were reported by KCSAN,

 write to 0xffff9275a1920ad8 of 16 bytes by task 47311 on cpu 46:
  xfs_vn_update_time+0x1b0/0x400 [xfs]
  xfs_vn_update_time at fs/xfs/xfs_iops.c:1122
  update_time+0x57/0x80
  file_update_time+0x143/0x1f0
  __xfs_filemap_fault+0x1be/0x3d0 [xfs]
  xfs_filemap_page_mkwrite+0x25/0x40 [xfs]
  do_page_mkwrite+0xf7/0x250
  do_fault+0x679/0x920
  __handle_mm_fault+0xc9f/0xd40
  handle_mm_fault+0xfc/0x2f0
  do_page_fault+0x263/0x6f9
  page_fault+0x34/0x40

 4 locks held by doio/47311:
  #0: ffff9275e7d70808 (&mm->mmap_sem#2){++++}, at: do_page_fault+0x143/0x6f9
  #1: ffff9274864394d8 (sb_pagefaults){.+.+}, at: __xfs_filemap_fault+0x19b/0x3d0 [xfs]
  #2: ffff9274864395b8 (sb_internal){.+.+}, at: xfs_trans_alloc+0x2af/0x3c0 [xfs]
  #3: ffff9275a1920920 (&xfs_nondir_ilock_class){++++}, at: xfs_ilock+0x116/0x2c0 [xfs]
 irq event stamp: 42649
 hardirqs last  enabled at (42649): [<ffffffffb22dcdb3>] _raw_spin_unlock_irqrestore+0x53/0x60
 hardirqs last disabled at (42648): [<ffffffffb22dcad1>] _raw_spin_lock_irqsave+0x21/0x60
 softirqs last  enabled at (42306): [<ffffffffb260034c>] __do_softirq+0x34c/0x57c
 softirqs last disabled at (42299): [<ffffffffb18c6762>] irq_exit+0xa2/0xc0

 read to 0xffff9275a1920ad8 of 16 bytes by task 47312 on cpu 40:
  xfs_vn_getattr+0x20c/0x6a0 [xfs]
  xfs_vn_getattr at fs/xfs/xfs_iops.c:551
  vfs_getattr_nosec+0x11a/0x170
  vfs_statx_fd+0x54/0x90
  __do_sys_newfstat+0x40/0x90
  __x64_sys_newfstat+0x3a/0x50
  do_syscall_64+0x91/0xb05
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

 no locks held by doio/47312.
 irq event stamp: 43883
 hardirqs last  enabled at (43883): [<ffffffffb1805119>] do_syscall_64+0x39/0xb05
 hardirqs last disabled at (43882): [<ffffffffb1803ede>] trace_hardirqs_off_thunk+0x1a/0x1c
 softirqs last  enabled at (43844): [<ffffffffb260034c>] __do_softirq+0x34c/0x57c
 softirqs last disabled at (43141): [<ffffffffb18c6762>] irq_exit+0xa2/0xc0

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 fs/xfs/xfs_iops.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 81f2f93caec0..2d5ca13ee9da 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -547,9 +547,9 @@
 	stat->uid = inode->i_uid;
 	stat->gid = inode->i_gid;
 	stat->ino = ip->i_ino;
-	stat->atime = inode->i_atime;
-	stat->mtime = inode->i_mtime;
-	stat->ctime = inode->i_ctime;
+	stat->atime = READ_ONCE(inode->i_atime);
+	stat->mtime = READ_ONCE(inode->i_mtime);
+	stat->ctime = READ_ONCE(inode->i_ctime);
 	stat->blocks =
 		XFS_FSB_TO_BB(mp, ip->i_d.di_nblocks + ip->i_delayed_blks);
 
@@ -614,11 +614,11 @@
 	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
 
 	if (iattr->ia_valid & ATTR_ATIME)
-		inode->i_atime = iattr->ia_atime;
+		WRITE_ONCE(inode->i_atime, iattr->ia_atime);
 	if (iattr->ia_valid & ATTR_CTIME)
-		inode->i_ctime = iattr->ia_ctime;
+		WRITE_ONCE(inode->i_ctime, iattr->ia_ctime);
 	if (iattr->ia_valid & ATTR_MTIME)
-		inode->i_mtime = iattr->ia_mtime;
+		WRITE_ONCE(inode->i_mtime, iattr->ia_mtime);
 }
 
 static int
@@ -1117,11 +1117,11 @@
 
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 	if (flags & S_CTIME)
-		inode->i_ctime = *now;
+		WRITE_ONCE(inode->i_ctime, *now);
 	if (flags & S_MTIME)
-		inode->i_mtime = *now;
+		WRITE_ONCE(inode->i_mtime, *now);
 	if (flags & S_ATIME)
-		inode->i_atime = *now;
+		WRITE_ONCE(inode->i_atime, *now);
 
 	xfs_trans_ijoin(tp, ip, XFS_ILOCK_EXCL);
 	xfs_trans_log_inode(tp, ip, log_flags);
-- 
1.8.3.1